Processes for refining the grain size of metals

ABSTRACT

In the grain refining of metals and alloys by introducing oxide particles into the molten metal to act as nuclei while it solidifies, the nuclei are introduced as internally oxidized particles of a metal carrier. Examples of such metal carriers are copper alloys containing 90 percent or more copper, the oxides in them being of nickel, cobalt or iron.

United States Patent Davies et a1.

[ 1 Jan. 18,1972

[54] PROCESSES FOR REFINING THE GRAIN SIZE OF METALS 22 Filed: Nov.13,1969

211 Appl.No.: 876,587

[30] Foreign Application Priority Data Nov. 21, 1968 Great Britain..55,328/68 [52] 'U.S. Cl ..75/135 [51] Int. Cl ..C22c 1/00 [58] Fieldof Search ..75/135, 162, 171, 144; 29/183.5, 191.6, 180 S, 159

[56] References Cited UNITED STATES PATENTS 480,445 8/1892 Whitney..75/144 1,147,398 7/1915 Henemier .....75/144 1,168,074 [[1916 Hunter..75/171 1,248,924 12/1917 Sandell ..75/159 2,285,308 6/1942 Specht...75/180 S 2,458,688 1/1949 Davis ...75/l59 3,303,531 2/1967 Ogden...75/171 3,360,366 12/1967 Bonis ..75/135 OTHER PUBLICATIONS Cibula, A.The Grain Refinement of Aluminum Alloy Castings by Additions of Titaniumand Boron." in Journal of the Institute of Metals, Vol. 80, 195 l- 1952,pp. 1- 16 Primary Examiner-Paul M. Coughlan, Jr. Assistant ExaminerG..1. Crasanakiis Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak 57ABSTRACT- In the grain refining of metals and alloys by introducingoxide particles into the molten metal to act as nuclei while itsolidilies, the nuclei are introduced as internally oxidized particlesof a metal carrier. Examples of such metal carriers are copper alloyscontaining 90 percent or more copper, the oxides in them being ofnickel, cobalt or iron.

8 Claims, No Drawings PROCESSES FOR REFINING THE GRAIN SIZE OF METALSThis invention relates to the refining of the grain size of metals oralloys during the process of solidification from the molten state.

It is known that reduction in the grain size can be etfected if solidparticles which act as nuclei are added to the molten metal. Out objectis to improve processes of this kind.

According to the invention the nuclei are introduced into a melt of themetal or alloy in a metal carrier that dissolves in the melt, beingpresent in the carrier as particles formed by internal oxidation ofmetal of the carrier and not greater than 2 microns, and preferably notgreater than 1 micron, in size. When the carrier is introduced into themelt and the metal of the carrier dissolves, the oxide particles areleft as a fine dispersion in the melt. Because of the small size of theparticles and the good wetting of the surfaces of the particles by themolten metal, the grain refining is excellent.

The reduction in the grain size depends upon the composition of thealloy, the size of the ingot or casting produced from the melt andotherfactors, but it has proved possible by means of the invention tocause an alloy which, without any addition of nuclei, would have had anaverage grain size of over 2 cm. to solidify with an average grain sizeof less than 1 mm.

To ensure that the nuclei become uniformly dispersed throughout themelt, it is important to prevent local chilling of the melt when themetal carrier is introduced. For this reason alone the carrier ispreferably of small section, and it is most advantageously strip or foilfrom 0.05 to 0.4 mm. thick. Another reason for using such foil is thatthe depth of penetration of the internal oxidation is a function of timeand temperature, and a metal carrier of greater thickness requires alonger time at the oxidizing temperature if it is to be uniformlyinternally oxidized. While it is not essential that the carrier shouldbe internally oxidized throughout, it is often better that it should be,so as to avoid the introduction of more of the metal of the carrier thanis necessary.

The choice of the alloy that is internally oxidized to form the metalcarrier depends on a number of factors. It must contain at least oneelement which is preferentially oxidized so that a dispersion of oxideparticles in metal can be produced. These particles must be effective asnuclei in the melt, and so must have melting points distinctly higherthan the temperature of the melt. The metal which remains unoxidizedmust readily dissolve in the melt, though it is not essential that it 1should have a melting point less than the temperature of the melt. Inthe amount added the unoxidized metal should not be a harmfulconstituent of the ingot or casting produced on the solidification ofthe melt. The alloy is preferably single-phase, since in a two-phasealloy the second phase may enter the melt as undissolved particleswhich, while not necessarily harmful, serve no useful purpose and mayinterfere with the nucleating process. I

With a suitable internally oxidized metal carrier, as little as 0.1percent carrier by weight of the melt will introduce enough nuclei to beeffective in refining the grain, but in general trot-r1002 to 0.5percent carrier by weight of the melt is a suitable amount The inventionis particularly useful in the treatment of melts of alloys having a baseof nickel, nickel and copper, nickel and chromium, or nickel, chromiumand cobalt, in each case with or without iron up to 50 percent. Suchalloys include those which have a base composed of from 20 to 80 percentnickel, 5 to 35 ercent chromium, 0 to 40 percent cobalt and 0 to 50percent iron, and which may also contain one or more other elements suchas aluminum or titanium up to percent each, and niobium tantalum,tungsten, zirconium, boron and hafnium in the amounts commonly presentin high-temperature alloys. For treating such melts, metal carriersconsisting of internally oxidized copper-nickel, copper-cobalt andcoppernickel-cobalt alloys containing by weight 90 percent or morecopper are suitable. Examples of composition are 95 percent copper and 5percent nickel; 96 percent copper and 4 percent cobalt; and 92.5 percentcopper, 5 percent nickel and 2.5 percent cobalt.

In melts which are refined and cast at lower temperatures, for examplemelts of copper-aluminum alloys, alloys containing iron as apreferentially oxidized element are suitable, examples being alloys ofcopper and iron or copper, iron and nickel, again containing at leastpercent copper. Examples of composition are percent copper and 5 percentiron and 92 percent copper, 4 percent iron and 4 percent nickel.

in the relatively small proportions in which the preferentially oxidizedelements are present in these examples of metal carriers, it is possibleto effect internal oxidation of most or nearly all of these elements ina reasonable time. Higher proportions may be used, but the rate ofinternal oxidation decreases, and in alloys containing, say, 30 percentof a preferentially oxidized element the oxide is not in the desiredsmall particle size. While the internal oxidation is most convenientlyeffected by air other atmospheres may be used, in particular oxygen.

The metal carrier may be externally as well as internally oxidized; forexample the surfaces of a carrier consisting predominantly of copper areconverted to copper oxide, with loss of weight of the carrier. It isdesirable to remove any surface oxide by brushing before the carrier isadded to the melt. In addition, the carrier may advantageously bedegreased before the addition.

Some examples of the production of a metal carrier will now be given.

EXAMPLE 1 Strip 0.2 mm. thick and 4 cm. wide of an alloy consisting of95 percent copper and 5 percent nickel was heated in air at 850 C. for 4hours. At the end of this time the strip was reduced in thickness to 0.1mm. as a result of oxidation of the surface, and substantially all thenickel in it was present as particles of nickel oxide, less than 1micron in size, uniformly dispersed throughout.

In contrast, heating more of the same strip in air at 600 C. results inoxidation predominantly or solely at the grain boundaries. Heating at700 C. requires a much longer time to produce adequate internaloxidation. Heating at 900 C. leads to undesirable coarser particles andheating at l,000 C. leads to oxidized particles averaging 20 microns insize, which are ineffective in reducing the grain size of the metal intowhich they are introduced.

EXAMPLE 2 Strip 0.2 mm. thick and 4 cm. wide of an alloy consisting of92.5 percent copper, 5 percent nickel and 2.5 percent cobalt was heatedin air at 850 C. for 4 hours. At the end of this time the strip wasreduced in thickness to 0.1 mm., and substantially all the nickel andcobalt in it were present as particles of nickel oxide and cobalt oxide,less than 1 micron in size, unifonnly dispersed throughout.

EXAMPLE 3 Strip'OA mm. thick and 6 cm. wide of an alloy consisting of 92percent copper, 4 percent nickel and 4 percent iron was heated in air at850 C. for 16 hours. At the end of this time the strip was reduced inthickness to 0.2 mm., and substantially all the nickel and iron in itwere present as particles of nickel oxide and iron oxide, less than 1micron in size, uniformly dispersed throughout.

EXAMPLE 4 Strip 0.2 mm. thick and 4 cm. wide of a two-phase alloyconsisting of 96 percent copper and 4 percent cobalt was heated in airat 850 C. for 4 hours. At the end of this time the strip was reduced inthickness to 0.08 mm. and the majority of the cobalt in it was presentas particles of cobalt oxide, less than 1 micron in size, uniformlydispersed throughout, but

second-phase particles of cobalt in it oxidized to give some particlesof cobalt oxide greater than 2 microns.

EXAMPLE 5 Strip 0.4 mm. thick and 6 cm. wide of a two-phase alloyconsisting of 95 percent copper and 5 percent iron was heated in air at850 C. for 16 hours. At the end of this time the strip was reduced inthickness to 0.2 mm. and the majority of the iron in it was present asparticles of iron oxide, less than 1 micron in size, uniformly dispersedthroughout, but secondphase particles of iron in it oxidized to givesome particles of iron oxide greater than 5 microns.

Some examples of the use of metal carriers will now be given.

EXAMPLE 6 A melt of 2,000 lb. of an alloy of nominal composition 92percent copper and 8 percent aluminum was made, and while it was at atemperature of 1,150 O, strips produced as described in example 5 andcontaining iron oxide particles were successively dropped onto thesurface of the melt and forced downwards by steel rods until a total of1.5 lb. of strip had been introduced. The melt was then cast into a sandmould with a carbon bottom to form slabs 20 cm. deep by 60 cm. by 60 cm.

The average grain size in the resultant slabs was less than 1 mm. Incontrast, when metal of the same nominal composition was cast into slabsin a similar manner, but without any nucleating addition, the averagegrain size was 2 cm.

EXAMPLE 7 A melt of 100 lb. of an alloy of nominal composition 92percent copper and 8 percent aluminum was made, and while it was at atemperature of l,150 C. strips produced as described in example 3,wrapped round 80/20 nickel-chromium wires of 2-mm. diameter, wereplunged below the surface of the melt until a total of 0.1 lb. of striphad been introduced. The melt was then cast into a metal mould 18 cm. indiameter. The average grain size in the resultant ingots was less than 1mm. In contrast, when metal of the same nominal composition was castinto ingots in a similar manner, but without any nucleating addition theaverage grain size was 1 cm.

EXAMPLE 8 A melt of lb. of an alloy of nominal composition 93 percentcopper and 7 percent aluminum was made and cast into a refractory mould7 cm. in diameter which had been preheated to l,000 C. When thetemperature of the molten metal in the mould had fallen to l,l30 C. apiece of strip weighing 0.01 lb., produced as described in example 5 andwrapped around 80/20 nickelchromiurn wire of l-mm. diameter, was plungedbelow the surface. The average grain size in the resultant ingot wasless than 1 mm. In contrast, when metal of the same nominal compositionwas cast into ingots in a similar manner but without any nucleatingaddition the average grain size was 0.6 cm. The plunging of anickel-chromium wire alone had no effect on grain size.

EXAMPLE 9 A melt of 10 lb. of an alloy of nominal composition 93 percentcopper and 7 percent aluminum was made and cast into a refractory mould7 cm. in diameter which had been preheated to 1,000 C. When thetemperature of the molten metal in the mould had fallen to 1,130 C. apiece of strip weighing 0.007 1b., produced as described in example 3and wrapped around 80/20 nickel-chromium wire of l-mm. diameter, wasplunged below the surface. The average grain size in the resultant ingotwas less than 1 mm. In contrast, when metal of the same nominalcomposition was cast into ingots in a similar manner but without anynucleating addition the average grain size was 0.6 cm.

EXAMPLE 10 A melt of 10 lb. of an alloy of nominal composition 88percent copper and 12 percent aluminum was made and cast into arefractory mould 7 cm. in diameter which had been preheated to 1,000 C.When the temperature of the molten metal in the mould had fallen to1,160 C. a piece of strip weighing 0.01 lb., produced as described inexample 4 and wrapped around /20 nickel-chromium wire of l-mm. diameter,was plunged below the surface. The average grain size in the resultantingot was less than 1 mm. In contrast, when metal of the same nominalcomposition was cast into ingots in a similar manner but without anynucleating addition the average grain size was 1.2 cm.

EXAMPLE 1 l EXAMPLE l2 Strips of total weight 0.01 lb. produced asdescribed in example l, wrapped round 80/20 nickel-chromium wire of 1-mm. diameter were plunged below the surface of a melt 10 lb. in weight.The alloy was then cast into a cast-iron mould 6 cm. in diameter. Theaverage grain size in the resultant ingot was less than 1 mm.

In contrast, when the same alloy was cast into ingots in a similarmanner but without any nucleating additions the average grain size was0.5 cm. The plunging of the wire alone had no effect on grain size.

EXAMPLE 13 Example 12 was repeated except that the total weight of thestrip was only 0.003 lb. Again, the average grain size in the resultantingot was less than 1 mm.

EXAMPLE 14 A melt of 20 1b. was made and strips of total weight 0.02lb., produced as described in example 1 and wrapped aroundnickel-chromium rod of S-mm. diameter were plunged below the surface ofthe melt to introduce nuclei of nickel oxide. The alloy was then castinto a refractory mould 7.5 cm. in diameter which had been preheated to850 C. The average grain size in the resultant ingot was less than 2 mm.

In contrast, when the same alloy was cast into ingots in a similarmanner but without any nucleating addition the average grain size was1.2 cm. The plunging of a rod alone had no effect on grain size.

EXAMPLE 15 The same result was obtained when example 14 was repeatedwith the substitution of strips produced as described in example 4 andso containing particles of cobalt oxide.

EXAMPLE 16 The same result was obtained when example 14 'was repeatedwith the substitution of strips produced as described in example 2 andso containing particles of nickel oxide and cobalt oxide.

We claim:

1. In a process in which the grain size of metal is refined duringsolidification from the molten state by the addition of solid nuclei toa melt of the metal, the improvement which comprises introducing saidnuclei into said melt in a metal carrier that dissolves in said melt,said nuclei being present in said carrier as oxide particles formed byinternal oxidation of metal of said carrier, said particles being notgreater than 2 microns in size.

2. A process according to claim 1 in which said nuclei are not greaterthan 1 micron in size.

3. A process according to claim 1 in which said metal carrier is in theform of strip from 0.05 to 0.4 mm. thick.

4. A process according to claim 1 in which the metal that is internallyoxidized to form said carrier is a single-phase alloy.

5. A process according to claim 1 in which said melt is an alloycomprising nickel and copper with from to 50 percent iron, and saidmetal carrier is formed by internal oxidation of an alloy containing atleast 90 percent copper, the remainder being metal selected from thegroup consisting of nickel and cobalt.

6. A process according to claim 1 in which said melt is an alloycomprising from 20 to 80 percent nickel, 5 to 35 percent chromium, 0 to40 percent cobalt and 0 to 50 percent iron, and said metal carrier isformed by internal oxidation of an alloy containing at least percentcopper, the remainder being metal selected from the group consisting ofnickel and cobalt.

7. A process according to claim 1 in which said melt comprises copperand aluminum which solidifies to form a singlephase alloy, and the metalof said carrier is formed by internal oxidation of an alloy containingat least 90 percent copper, the remainder being metal selected from thegroup consisting of iron, nickel and cobalt.

8. A process according to claim 1 in which said melt comprises copperand aluminum which solidifies to form an alloy containing thebeta-phase, and the metal of said carrier is formed by internaloxidation of an alloy containing at least 90 percent copper, theremainder being metal selected from the group consisting of iron, nickeland cobalt.

2. A process according to claim 1 in which said nuclei are not greaterthan 1 micron in size.
 3. A process according to claim 1 in which saidmetal carrier is in the form of strip from 0.05 to 0.4 mm. thick.
 4. Aprocess according to claim 1 in which the metal that is internallyoxidized to form said carrier is a single-phase alloy.
 5. A processaccording to claim 1 in which said melt is an alloy comprising nickeland copper with from 0 to 50 percent iron, and said metal carrier isformed by internal oxidation of an alloy containing at least 90 percentcopper, the remainder being metal selected from the group consisting ofnickel and cobalt.
 6. A process according to claim 1 in which said meltis an alloy comprising from 20 to 80 percent nickel, 5 to 35 percentchromium, 0 to 40 percent cobalt and 0 to 50 percent iron, and saidmetal carrier is formed by internal oxidation of an alloy containing atleast 90 percent copper, the remainder being metal selected from thegroup consisting of nickel and cobalt.
 7. A process according to claim 1in which said melt comprises copper and aluminum which solidifies toform a single-phase alloy, and the metal of said carrier is formed byinternal oxidation of an alloy containing at least 90 percent copper,the remainder being metal selected from the group consisting of iron,nickel and cobalt.
 8. A process according to claim 1 in which said meltcomprises copper and aluminum which solidifies to form an alloycontaining the beta-phase, and the metal of said carrier is formed byinternal oxidation of an alloy containing at least 90 percent copper,the remainder being metal selected from the group consisting of iron,nickel and cobalt.